Dehydration (hypohydration) is a condition that occurs when the loss of body fluids, mostly water, exceeds the amount that is taken in. More water is moving out of the cells and bodies than what an individual takes in through drinking. With dehydration, the excessive loss of body water is accompanied by disruption of metabolic processes
The term dehydration can refer to the following conditions:                hypernatremia which is defined by an elevated sodium level in the blood (loss/deficit of free water and the attendant “excess” concentration of salt). It is related to a disruption of the body's electrolyte-water balance (osmolarity).        hypovolemia which is a state of decreased blood volume; more specifically, decrease in volume of blood plasma (loss of blood volume, particularly plasma). The volume loss can be isotonic and preserve the electrolyte-water balance or not (for instance hypotonic state: salt depletion). Hypovolemia is reported to be the most common dehydration form.        
Hypernatremia and hypovolemia can co-exist or occur independently; therefore it is important that the measurement principle of the hydration can cover both of them. Particularly, the method used for hydration estimation should be able to quantify the body water loss with an order of magnitude of a few percents variations that reflect different levels of dehydration and different symptoms (see Table below).
Body water lossSymptomsLevel of dehydration  >2%Threshold: dehydrationMinimalstarting>3-4%Tolerable dehydration, noMildcritical symptoms>5-8%Fatigue & dizzinessModerate >10%Physical and mentalSeveredeterioration, severe thirst>15-25% LethalVery severe
According to the French National Institute of Health, in 2003 the heat wave in France caused 14′802 heat-related deaths, mostly among the elderly. The number of annual deaths in the UK resulting from the heat is expected to rise by 257% by 2050. The main cause of these worrying figures is acute dehydration.
In sports, dehydration during intense effort and under hot environment conditions leads to reduced performance.
Body mass is often used to assess the rapid changes of hydration in both laboratory and field environments. The level of dehydration is expressed as a percentage of starting body mass. There is evidence that body mass may be a sufficiently stable physiological marker for monitoring daily fluid balance, even over longer periods (1-2 weeks). Over longer periods, changes in body composition (fat and lean mass) are also reflected grossly as changes in body mass, thus limiting this technique for assessment of hydration. This technique is not 100% robust and cannot be used for a real-time assessment of dehydration. Experts acknowledge the difficulty in attempting to assess this true hydration and promote the use of a number of the more established methods to ensure the best representation of hydration status.
There is a stringent demand in terms of wearable systems able to non-invasively evaluate in real time and autonomously the hydration level and the related critical thresholds of a person in various life scenarios.